An NMR apparatus traditionally includes means for subjecting a body to a constant and intense magnetic field. Thus conditioned, some regions of the body may be energized by radiofrequency excitation and, when this excitation ceases, induce a resonance signal which is measured and which includes information useful for creating images of parts of the body. The excitation and measuring means include a radiofrequency antenna; this antenna is disposed about the body to be examined. The load seen by this antenna at the time of emission, or which comes to the same thing, the internal impedance of the generator which it forms at the time of the measurement, depends essentially on the body subjected to the examination. From one body to another it changes. To provide the best excitation and the best measurement, it is necessary to tune the antenna. Furthermore, the NMR antenna must be matched to the high frequency line which connects it, through a duplexer, either to the emitter or to the receiver. An adjustable device must then be associated with the antenna for matching its impedance. In practice, since high frequency lines are coaxial cables characterized by their characteristic impedance, for example of 50 ohms, the antenna must further be adjusted so that it presents such a real impedance at the operating frequency.